Pressure polishing of extruded polycarbonate or polysulfone sheet

ABSTRACT

Pressure polished polycarbonate or polysulfone sheet substantially free of ripples, wiggles, extrusion die lines, and stresses and having a light transmittance greater than, a percentage haze lower than and a distortion lower than the as extruded sheet so that the polished sheet can be mechanically or thermo-formed into such articles as airplane windshields, canopies, protective headgears and see-through enclosures requiring minimal wall thickness, and a method of polishing the sheet between mirror finish plates using a combination of preheating to the heat distortion temperature at zero or minimal pressure, heating to higher temperatures while rapidly increasing the pressure so as to transfer the mirror finish from the plates to the sheet surfaces, and cooling the sheet while decreasing the pressure to fix the mirror finish in the sheet surfaces as the sheet contracts and solidifies.

Unite States ate t Moore PRESSURE POLISHING OF EXTRUDED POLYCARBONATE ORPOLYSULFONE SHEET CHARGING PRESS PLACING PLAST SHEET IN PRESS BETWEENTWO MIRROR FINlSI-I PLATES H EATING CYCLE (II) FOREIGN PATENTS ORAPPLICATIONS 1,069,744 1967 Great Britain ..264/235 PrimaryExaminer-Robert F. White Assistant Examiner-Richard R. KuciaAttorney-McClure & Millman 5 7] ABSTRACT Pressure polished polycarbonateor polysulfone sheet substantially free of ripples, wiggles, extrusiondie lines, and stresses and having a light transmittance greater than, apercentage haze lower than and a distortion lower than the as extrudedsheet so that the polished sheet can be mechanically or thermo-formedinto such articles as airplane Windshields, canopies, protectiveheadgears and see-through enclosures requiring'minimal wall thickness,and a method of polishing the sheet between mirror finish plates using acombination of pre-heating to the heat distortion temperature at zero orminimal pressure, heating to higher temperatures while rapidlyincreasing the pressure so as to transfer the mirror finish from theplates to the sheet surfaces, and cooling the sheet while decreasing thepressure to fix the mirror finish in the sheet surfaces as the sheetcontracts and solidifies.

9 Claims, 6 Drawing Figures PREHEATING CYCLE CLOSING PRESS ANDMAINTAINING PRESSURE AT 0 T050 PSI (M X) WHILE SLOWLY RAIS- INc TEMI. OFSANDWKH T0 HEAT. DISTORTION TEMP. OF PLASTK HEATING CYCLE (1) RAISINGTEMP T0 ABOUT RAISING TEMP. TO ABOUT 350 TO 360F (MAX) 310 TO 330 FWHILE SLOWLY REDUC- WHILE RAPIDLY IN- ING PRE SURE T CREASING PRESSUREABOUT ONE HALF ITS T0 ABOUT500-5OO PSI ORIGINAL VALUE (FORMING MIRROR(Ioo Iso PsI) SURFACE IN PLASTIC) COOLING CYCLE COOLING SLOWLY TO ABOUT150 F WHILE MAINTAINING PRESSURE AT ABOUT I00- I50 PSI (soLIoIFYINo)REMOVING POLISHED POLYCARB OPENING PRESS TO REMOVE POLISHED PLASTICSHEET ALLOWING TO COOL IN ATMOS HERE PATENTEYDAUE 3 m2 SHEET 10F 2wean/70x RICHARD E. MOORE PATENTEDIIII: I I972 CHARGING PRESS SHEET 2 OF2 PLACING PLASTIC SHEET IN PRESS BETWEEN TWO MIRROR FINISH PLATES I-IEATING was (11) PRE-HEATING CYCLE T0 HEAT DISTORTION CLOSING PRESS ANDMAINTAINING PRESSURE AT 0 T050 PSI (MAX) WHILE SLOWLY RAIS- ING TEMI. OFSANDwIfl-I TEMP. OF PLASTK HEATING CYCLE (I) RAISING TEMP. TO ABOUTCREASING PRESSURE COOLING CYCLE COOLING SLOWLY TO ABOUT I5o F WHILEMAINTAINING PRESSURE AT ABOUT I00- I50 PsI (SQLIDIFYIN 3Io TO 330 FWHILE RAPIDLY IN- TO ABOUT 3oo-5oo PSI (FORMING MIRROR SURFACE INPLASTIC) REMOVING POLISHE D POLYCARB ALLOWING TO COOL OPENING PRESS TOREMOVE POLISHED PLASTIC SHEET IN ATMOSPHERE M/Vl-Wffi RICHARD E. MOOREB) QIWMQ 5 $05M PRESSURE POLISHING OF EXTRUDED POLYCARBONATE ORPOLYSULFONE SHEET This invention relates to optical grade polycarbonateor polysulfone sheet and a method of pressure polishing the extrudedsheet to produce the optical grade material.

Polycarbonate resins are polyesters of carbonic acid and bisphenol A,are transparent and are well known for their high mechanical strengthproperties such as toughness, high impact strength and high tensilestrength over a broad temperature range and non-shatteringcharacteristics. They are extruded as sheets and as such contain ripplesand extrusion die lines or stresses which cause undesirable opticalproperties such as relatively low luminous transmittance and a highpercentage of haze. Polycarbonates do not lend themselves to removal ofthese ripples, blemishes, die lines, scratches or nicks by conventionalgrinding, buffing and polishing techniques.

Polysulfone resins are thermoplastic polymers composed of phenyleneunits linked by three different chemical groups, namely isopropylidene,ether and sulfone whose most distinctive feature is the diphenylenesulfone group. They are thermally stable, resistant' to oxidation, rigidat elevated temperatures, tough, and have high impact and tensilestrength over a broad temperature range. They are commercially availablein many grades from companies such as Union Carbide. The sheets ofpolysulfone are generally extruded. Like the polycarbonates, the ripplesand extrusion die lines or stresses are difficult to remove byconventional grinding and buffing techniques.

It is the primary object of this invention to provide an optical gradepolycarbonate or polysulfone sheet having luminous transmittance betterthan, haze less than and distortion less than the as extruded sheet.Such material, being tougher than acrylics, vinyls, celluloses and otherclear thermoplastics, and having desirable optical properties, canadvantageously be used to fabricate, mechanically or by thermo-forming,airplane Windshields, canopies, protective headgear and seethroughenclosures requiring minimal wall thicknesses and the like.

Another object of the invention is to provide an efficient, accurate andreproducible method of making polycarbonate or polysulfone sheets withoptical properties superior to the as extruded sheets using controlledstages of heat and pressure and mirror polished plates to eliminatesubstantially all of the ripples, wiggles and extrusion die lines andstresses, which produce the inferior optical properties of the asextruded sheets.

Another object of the invention is to provide a method of pressurepolishing extruded polycarbonate or polysulfone sheets employing asequence of first charging a press with the extruded sheet retainedbetween plates of high mirror polish, then pre-heating the sheet slowlyto its heat distortion temperature with little or no pressure applied soas to expel air entrapped between the sheet and the mirror polishedplates, then rapidly raising the pressure to about 300-500 psi while thetemperature rises to the fusion temperature at which time mirrorsurfaces are formed in the sheet and the ripples, wiggles and extrusiondie lines and stresses are removed, and finally cooling the materialslowly to about 150 F while cutting the pressure in half to retain themirror polish on its surfaces while the material cools and solidifies.If the polycarbonate or polysulfone will eventually be thermo-formedinto its final article, to eliminate molecular memory in which thesurfaces of the plastic sheet may revert to their original formincluding the ripples, wiggles and extrusion die lines or stresses, thestep of raising the temperature somewhat higher than the fusiontemperature while reducing the pressure to prevent thinning isinterposed between the fusion stage and the cooling stage.

These and other objects of the invention will become more apparent asthe following description proceeds in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a diagrammatic view of a multi-platen press shown open andcharged with the members required to initiate pressure polishing of thepolycarbonate or polysulfone;

FIG. 2 is a view similar to FIG. 1 showing the press closed;

FIG. 3 is a sectional view through the unit to be charged into thepress;

FIG. 4 is a perspective view of the final highly polished plastic sheet;

FIG. 5 is a sectional view of a modified unit to be charged into thepress; and

FIG. 6 is a flow diagram of the sequential process steps.

Specific reference is now made to the drawings in which similarreference characters are used for corresponding elements throughout.

Polycarbonate resins which are polyesters of carbonic acid and bisphenolA are available in various grades commercially under such trademarks asLexan (General Electric), Merlon (Mobay Chemical), etc. They have hightensile and impact strength over broad temperature range, are virtuallyshatter-proof and are extrudable as transparent sheets. Since the sheetscontain ripples, wiggles, and extrusion die lines or stresses they donot possess desirable optical properties. Moreover, they cannot bereadily ground, buffed and polished by conventional means to removethese ripples, die lines and other flaws.

Polysulfone resins are polymers composed of phenylene units linked byisopropylidene, ether and sulfone groups which have high impact andthermal strength and are extrudable as transparent sheets and asextruded contain ripples, wiggles and extrusion die lines or stressesthat impair the optical properties of the sheet and are not readilypolished by conventional grinding and buffing techniques.

The steam heated press 10 employed to carry out the process isconventional and generally comprises a head 12 having an upper platen 14at its lower surface, a vertically movable bed 16 carrying a lowerplaten l8 and a plurality of vertically movable platens 20 (only oneshown as illustrative) between the upper and lower platens definingbetween them the chargingand polishing positions. The platens includeinterior coils which are connected by suitable couplings and flexiblehoses 22 to a source S of steam under pressure via a valve V there beinga pressure gauge 24 in the line. The platens are also connected bysuitable couplings and flexible hoses 26 to a source of cold water CWvia an appropriate valve V The press is hydraulically operated frombelow by means of a ram 28 secured to the bed 16 and vertically movablein a cylinder 30. Appropriate conduits 32 connect the cylinder to asource of hydraulic fluid R via a pump P and a pressure gauge 34. Themeans to raise and lower the ram and thus close and open the press arewell known and can involve a reversible pump P or a pair of pumps andseparate hydraulic circuitry or equivalent means.

When the ram is retracted and the press is open as shown in FIG. 1, unit36 of FIG. 3 is charged therein by placing each unit on a platen spacedfrom the adjacent upper platen. The unit 36 comprises an extruded sheet38 of polycarbonate or polysulfone, a pair of upper and lower mirrorpolished plates 40 coextensive with and in face to face contact with theopposite surfaces of the plastic, a pair of upper and lower aluminumbacker plates 42 in face to face contact with the mirror polished platesand a pair of upper and lower cardboard sheets 44 in face to facecontact with the aluminum plates. When the unit is positioned in thepress the lower cardboard object contacts one surface of each platen andwhen the press is closed, as shown in FIG. 2, the upper cardboard sheetof that unit 36 contacts the undersurface of the next upper platen.

The plate 40 is preferably a hard rolled brass press plate having apressure resistance of about 64,000 psi, offering excellent resistanceto deformation, made from a double spring hard special copper alloy witha clean alpha structure, i.e., only one kind of crystal, and istherefore capable of taking a mirror-like polish (National ElectricalAssociation of the USA designation No. 8) without any visible polishinglines. To protect the mirror polished surface, the plate may be nickelplated with a layer of about 0.00032 inch or plated on the nickel with alayer of about 0.00008 inch chromium. Such a plate is commerciallyavailable as Wieland highly mirror polished press plates sold by FabriteMetals Corporation, New York, New York. In place of the chrome mirrorplate, a heat or chemically tempered glass with highly polished parallelsurfaces can be used, or any other equivalent mirror finish plate,provided the same can withstand the pressures and temperatures employedin the process and any torque to which the plate may be subjected due touneven closure of the platens.

The aluminum backer plates 42 are employed to protect the mirrorpolished plates 40 as well as to distribute the heat evenly over theplastic sheet 38 and to reduce imperfections which may exist in theflatness of the platens.

The cardboard sheets 44 are somewhat resilient and are used as goodinsulators to even out the heating and cooling of the plastic sheet 38and to help level out high and low spots and thus prevent blisters.

Although the unit 36 is shown to be of lesser width than the platens,this is merely for illustrative purposes. The unit can desirably becoextensive with the platen.

For the pressure polishing of polycarbonate or polysulfone sheets up tothree-eighths inch, there is not sufficient lateral flow under theconditions of pressure and temperature used in the process, as willappear hereinafter, to require that the plastic be laterally confinedand hence the unit 36 will be charged into the press as such. However,such lateral flow may be encountered when the thickness of the sheet tobe polished exceeds three-eighths inch, in which case lateralconfinement is required and for this unit 46 shown in FIG. 5 is used.Thus the lower aluminum plate 48 is the base plate of a mold upon whichan aluminum frame 50 is secured to provide a mold cavity 52 which isnarrowed by the insertion of a frame 54 of compressible wood, such aspine, which extends above the aluminum frame 50. The lower mirrorpolished plate 56 is laid in the cavity against the base plate 48 andwithin the confines of the wooden frame 54 and the polycarbonate orplastic sheet 38 is placed on the lower mirror polished plate 56 alsowithin the confines of the wooden frame. The thickness of the plasticsheet is such that its upper surface is beneath the upper edge of thewooden frame upon which is placed the upper mirror polished plate 58.Over the upper mirror polished plate is placed the upper aluminum backerplate 60. It is to be understood that when the unit 46 is charged intothe press, upper and lower cardboard sheets 44 will be placed over theupper aluminum plate 60 and below the aluminum base plate 48respectively.

The units 36 are charged in the press, and while only two such units areillustrated, it will be understood that many more can be employed forthe simultaneous pressure polishing without impairing the efficiency andaccuracy of the process. The press is then closed by activation of thepump P so that the ram 28 raises the bed 16 and all the platens save theupper 14, in which closure action the upper cardboard sheets engage theundersurface of adjacent upper platens in each operative station, asseen in FIG. 2. Steam is admitted to the platens via the valve V and thetemperature is raised slowly while the pressure is maintained at zero(weight of the platens) up to 50 psi on the plastic sheet 38 as amaximum. The temperature to which the plastic sheet is raised is itsheat distortion temperature which, by definition, is the temperature ofthe material when it begins to distort under a pressure of 66 psi. Inthe case of polycarbonate this temperature is about 270 F and the periodover which such temperature is attained is about 30 minutes for one-halfto three-fourths inch material and in the case of polysulfone is about320 F and is reached in about 45 minutes for similar thicknesses. Inthis pre-heat stage or cycle entrapped air is expelled between themirror polished plates 40 and the aluminum plates 42 and causes evenface to face contact of the mirror polished plates and the matingsurfaces of the plastic sheet.

Where the unit 46 is used to laterally confine the plastic sheet at itsheat distortion temperature, the closing of the press causes the woodenframe 54 to com press and the upper mirror polished plate 58 to contactthe upper surface of the plastic sheet. The upper mirror In the next orheating stage, the pressure is raised rapidly to about 300 to 500 psiwhile the temperature is allowed to increase to about 3lO-330 F. Thistemperature rise is obtained over a period of to 20 minutes while thepressure is maintained at said value of 300 to 500 psi, depending uponthe thickness of the plastic sheet. In this heating stage or cycle themirror polish of the plates 40 is transferred to the plastic sheet whilethe ripples, wiggles, and extrusion die lines and stresses aresubstantially eliminated.

If the polished polycarbonate or polysulfone is destined forthermo-forming into final articles, then a second heating stage or cycleis used to destroy the molecular memory of the material so that theoriginal ripples, wiggles, and extrusion die lines and stresses will notreturn upon fabrication. In this molecular memorydestroying cycle thematerial is raised to a temperature of about 350360 F while the pressureis slowly reduced to about half its original value as a maximum, andpreferably 50 to 75 psi, over a period of about 20 minutes. Thisdecreased pressure prevents excessive fiow and thinning of the plasticsheet.

Thereafter the material is cooled slowly to about 150 F whilemaintaining the decreased pressure at a maximum of 150 psi andpreferably 50 to 75 psi. The cooling stage is effected by shutting valveV and opening valve V to the cold water supply CW while maintaining thedesired ram pressure. The cooling cycle is about 45 minutes. In thefirst several minutes thereof this low pressure is maintained, that isuntil the temperature drops about 10 F. Thereafter, the pressure on thematerial can be allowed to rise while the ram pressure control is turnedoff so that as the plastic sheet cools and contracts, the press pressurewill drop accordingly.

If the polished polycarbonate or polysulfone will not be destined forfabrication into a final article by thermo-forming, then the secondheating stage or cycle can be eliminated and the material which wassubjected to a temperature of about 310-330 F and a pressure of 300 to500 psi is subjected directly to the cooling cycle.

After the press is opened and the polished plastic sheet is removed, itis allowed to cool slowly in the atmosphere. Because the surfaces of thepressure polished polycarbonate and polysulfone sheets are substantiallyfree of ripples, wiggles and extrusion die lines or stresses, thepolished sheets have greater luminous transmittance, lower haze andlesser optical distortion than the as extruded sheets and thus suchpressure polished sheets can be used advantageously to fabricate sucharticles as airplane Windshields, canopies, protective headgear andother enclosures requiring good seethrough properties but minimal wallthickness. In the case of polycarbonate which was pressure polished bythe process of the instant invention, the following comparative data wasdetermined by ASTM Procedure D 1003-61 using one-eighth inch polishedpolycarbonate, one-eighth inch as extruded polycarbonate and oneeighthinch glass:

Polished As Extruded Haze 0.7 1.6 0.0

Additionally to determine optical distortion, a grid pattern (1 inchsquares) was photographed through optical flat glass, common windowglass, standard extruded polycarbonate sheet and pressure polishedpolycarbonate sheet using a commercial 14 inch camera, f/6.3 stoppeddown to f/4.5 2 l kw scoop lights, a camera to sample distance of 5 feetand a samplc to grid distance of l 1 feet. The results were that thepressure polished polycarbonate had considerably less distortion thanthe window glass but was not quite as distortion-free as the opticalflat glass.

Thus it will be seen that the pressure polished polycarbonate hasoptical properties superior to those of the as extruded sheet andoptical distortion approaching that of optical flat glass providingdesirable see-through clarity and lack of distortion for manyapplications. It should be noted that the superior optical propertiesalso result from the fact that the outer surfaces of the sheet arerendered substantially parallel during the pressure polishing thereof.Additionally, the instant invention makes possible the formation ofpressure polished polycarbonate sheets which exceed in thickness that ofthe as extruded polycarbonate sheets, which at present are limited tothree-eighths inch.

While preferred embodiments of the invention have here been shown anddescribed, it will be understood that minor variations may be madewithout departing from the spirit of the invention.

What is claimed is:

l. A method of pressure polishing an extruded transparent unfilled sheetof polycarbonate or polysulfone resins having ripples, wiggles,extrusion die lines and stresses comprising first slowly pre-heating thesheet between a pair of mirror polished plates at a pressure of zero toa maximum of 50 psi to the heat distortion temperature of the sheet toexpel air entrapped between the sheet and the plates, then rapidlyincreasing the pressure to about 300 to 500 psi while allowing thetemperature to rise to about 310 F to transfer the mirror finish of theplates to the sheet surfaces while substantially eliminating theripples, wiggles, extrusion die lines and stresses, and then slowlycooling the sheet while reducing the pressure to about to psi and thetemperature to about 150 F to fix the mirror finish in the sheetsurfaces as the sheet contracts and solidifies.

2. The method of claim 1 wherein the sheet is polycarbonate and the heatdistortion temperature in the pre-heat stage is about 270 F.

3. The method of claim 1 wherein the sheet is polysulfone and the heatdistortion temperature in the pre-heat stage is about 320 F.

4. The method of claim 1 wherein the mirror polished plates are made ofspring hard brass having a clean alpha crystal structure polished sothat no polishing lines are visible.

5. The method of Claim 4 wherein the mirror polished brass plates arenickel or nickel-chrome plated.

6. The method of claim 1 wherein the sheet is at least three-eighthsinch thick and the step of confining the sheet laterally throughout theentire process to restrain lateral flow.

7. A method of pressure polishing an extruded transparent unfilled sheetof polycarbonate or polysulfone resin having ripples, wiggles, extrusiondie lines and stresses comprising first pre-heating the sheet between apair of mirror polished plates at a pressure of zero to a maximum of 50psi to the heat distortion temperature of the sheet to expel airentrapped between the sheet and the plates, then increasing the pressureto about 300 to 500 psi while allowing the temperature to rise to about3l0-330 F to transfer the mirror finish of the plates to the sheetsurfaces and substantially eliminating the ripples, wiggles, extrusiondie lines and stresses, then reducing the pressure to about 100 to 150psi while elevating the temperature to a maximum of 360 F to destroy themolecular memory of the material, then cooling the sheet while reducingthe pressure to fix the minor finish in the sheet surfaces as the sheetcontracts and solidifies and thermoforming said sheet.

8. The method of claim 7 wherein the pressure in the cooling stage isdecreased to about to psi and the temperature to about 150 F.

9. The method of claim 8 wherein the temperature in the pre-heat stageis slowly raised to the heat distortion temperature, then the pressureis rapidly increased to about 300 to 500 psi in the next heating stageand the sheet is slowly cooled in the cooling stage.

223 UNITED STATES PATENT OFFICE CERTIFICATE ()F CORRECTI@N mantis.3,6815483 Dated August 1, 1972 lnventofls) Richard E. Moore It iscertified that error appears in the above-identified patent and that.said Letters Patent are hereby corrected as shown below:

Col. 6, line 43, after 310 insert "-330 F Signed and sealed this'asthday of December 1972. v

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GO'ITSCHALK A-ttesting Officer Commissionerof Patent;

2. The method of claim 1 wherein the sheet is polycarbonate and the heatdistortion temperature in the pre-heat stage is about 270* F.
 3. Themethod of claim 1 wherein the sheet is polysulfone and the heatdistortion temperature in the pre-heat stage is about 320* F.
 4. Themethod of claim 1 wherein the mirror polished plates are made of springhard brass having a clean alpha crystal structure polished so that nopolishing lines are visible.
 5. The method of Claim 4 wherein the mirrorpolished brass plates are nickel or nickel-chrome plated.
 6. The methodof claim 1 wherein the sheet is at least three-eighths inch thick andthe step of confining the sheet laterally throughout the entire processto restrain lateral flow.
 7. A method of pressure polishing an extrudedtransparent unfilled sheet of polycarbonate or polysulfone resin havingripples, wiggles, extrusion die lines and stresses comprising firstpre-heating the sheet between a pair of mirror polished plates at apressure of zero to a maximum of 50 psi to the heat distortiontemperature of the sheet to expel air entrapped between the sheet andthe plates, then increasing the pressure to about 300 to 500 psi whileallowing the temperature to rise to about 310*-330* F to transfer themirror finish of the plates to the sheet surfaces and substantiallyeliminating the ripples, wiggles, extrusion die lines and stresses, thenreducing the pressure to about 100 to 150 psi while elevating thetemperature to a maximum of 360* F to destroy the molecular memory ofthe material, then cooling the sheet while reducing the pressure to fixthe mirror finish in the sheet surfaces as the sheet contracts andsolidifies and thermoforming said sheet.
 8. The method of claim 7wherein the pressure in the cooling stage is decreased to about 100 to150 psi and the temperature to about 150* F.
 9. The method of claim 8wherein the temperature in the pre-heat stage is slowly raised to theheat distortion temperature, then the pressure is rapidly increased toabout 300 to 500 psi in the next heating stage and the sheet is slowlycooled in the cooling stage.